This invention relates to switched mode power supplies, and self-oscillating power converters in particular. The invention is particularly applicable, but by no means limited, for use in fluorescent lighting ballasts.
Bipolar junction transistors (BJTs) can be used as the switches in switched mode power supplies (SMPSs) due to their low cost. The SMPSs using them are normally built to be self-oscillating using inductors and capacitors, needing no external control to cause the switching except when starting the SMPS. In these SMPSs, a transformer having a winding connected in series with the BJT's load current provides the base current of the BJT through a secondary winding. These SMPSs can be used as electronic ballasts in fluorescent lighting.
FIG. 1 is a schematic circuit diagram of the basic components of a self-oscillating electronic ballast 2 based around BJTs. NPN BJTs 4 and 6 are connected in series so as to provide a midpoint 8 between voltage rail 10 and voltage rail 12. A load winding 14 is connected between the midpoint 8 and the load 16. Drive windings 18 and 20 are connected to the base terminals of BJTs 4 and 6 respectively, and are wound on the same core as load winding 14 such that the three windings around the core act as a transformer. The small dots indicate the alignment of the windings with respect to each other on the core in the usual manner. The BJTs are connected in parallel with freewheeling diodes 22 and 24, which allow load current to flow during the period in which the BJTs have switched but the current has not yet commutated. Alternatively, the freewheeling diodes may be connected to the bases of BJTs 4 and 6. The load 16 may include reactive, capacitive and/or resistive components. The circuit is generally completed by connections (not shown) from the load 16 to the low voltage rail 12 and/or the high voltage rail 10. A common variation on this circuit includes the use of a resistor in series with each BJT so as to assist in turning the BJTs 4 and 6 off in high current conditions. Other circuits include resistors connected in series with the bases of the BJTs 4 and 6 to help control the frequency of oscillation.
Generally, the circuit is initiated by supplying a large current to the base of BJT 6 so as to rapidly turn it fully on. Means for doing this commonly use a DIAC and are not discussed here. When BJT 6 is triggered, the voltage at midpoint 8 rapidly reaches that of the low voltage rail 12, causing current to flow through the load 16. As this occurs, current flows through load winding 14 away from the dot, causing current in drive winding 20 to flow towards the dot—providing more current to the base of BJT 6 and keeping it on. At the same time, current in drive winding 18 flows towards the dot, hence drawing current away from the base of BJT 4 and holding it off.
As the load current through load winding 14 increases, the magnetisation current also increases, leading to a current in drive winding 20 that does not increase as rapidly as the load. This means that the base current in BJT 6 is also not increasing as rapidly as the load. Eventually the ratio of load current to base current will exceed the gain of the BJT 6, this causes BJT 6 to start to turn off. As BJT 6 turns off, current through it is restricted and so current through load winding 14 starts to pass through freewheeling diode 22. The load current starts to decrease and eventually the load current commutates, causing current in winding 14 to reverse. The current in drive winding 20 now flows away from the base of BJT 6 and current in drive winding 18 flows towards the base of BJT 4, causing it to start conducting and allowing current to flow from the load to the high voltage rail 10. Eventually BJT 4 shuts down as BJT 6 did, and the current passing through the load commutates again and BJT 6 starts conducting again.
This self-oscillation occurs at a frequency that depends on the properties of the components, such as inductors, resistors and transistors in the circuit. However, it is difficult and expensive to accurately control the tolerances of these devices. This results in an unpredictable self-oscillating frequency that differs from circuit to circuit and may be too high or low for the required task.
One common change that has been made to electronic ballasts in recent years has been the move to the use of field effect transistors (FETs) rather than BJTs. FETs offer greater control than BJTs due to the fact that a voltage, rather than a current, activates them. This has led to their growing adoption in the integrated ballasts of compact fluorescent lamps (CFLs), as this ease of control allows the production of CFLs with improved start characteristics and longevity. However, FETs are significantly more expensive than BJTs, and this use of FETs increases the costs associated with the manufacture of CFLs.
It is an object of this invention to provide a device and method for better controlling the BJTs used in self-oscillating power converters.